The need for nondestructive detection of defects in piping is necessary to avoid costly shut down of equipment and for ensuring the integrity of aged piping, specifically where, the aged piping is carrying high pressure combustible fluids and gases that are a safety hazard. The defects detected may include but are not limited to flaws such as cracks, dents and pits.
One embodiment of such a system uses multiple ultrasonic piezoelectric transducers mounted inside a liquid filled elastomer "wheel." This system transmits an ultrasonic wave that is refracted at the elastomer/pipe wall interface and is subsequently guided by the pipe wall.
When the compressional wave (the p-wave) produced by the transducer and, transmitted by the liquid in the wheel, reaches the interface between the wheel elastomer and the steel pipe wall, some of the p-wave energy is converted to vertically polarized shear waves (Sv waves) and continues to travel in the pipe wall until it encounters some type of defect within the pipe wall. The defect may be a crack, an inclusion, a lamination, or other defect. When the Sv wave encounters the defect, a portion of the energy is reflected back along the same raypath to the transducer which, in the meantime, is converted from a transmitter to a receiver. The reflected signal is then indicated by an electrical pulse or waveform at the transducer terminals. The initial processing is simiplistic; if no signal is received during the "on time", which is the time that the transducer functions as a receiver, then the decision is "no defects in the raypath" for that segment of the pipeline. However, if a reflected signal is received, then the decision is "a defect present in the raypath" of the Sv wave. At this point, the process becomes more complex; the user must now decide if the defect detected is one of interest such as a crack or something relatively benign such as a lamination. Considerable effort has gone into the process of discriminating specific types of defects. Unfortunately, only one company has claimed satisfactory success in defect discrimination and the details remain unavailable to those outside that organization.
A second embodiment utilizing ultrasound also employs piezoelectric transducers. However, instead of using a liquid filled wheel to "couple" the ultrasound to the pipe wall, the line is flooded with a suitable liquid in order to provide a means for transferring the ultrasonic energy into the steel pipe wall. Moreover, in order to make the system effective for crack detection, the transducers are mounted at an angle with respect to a plane tangent to the pipe wall at a point directly over the transducer in the radial direction. Consequently, the identical effect discussed in the previous embodiment is similarly implemented. Namely, the ultrasonic p-wave travels through the liquid medium and strikes the pipe wall at the Brewster angle. When this happens, part of the incident energy is converted to an Sv wave and continues to travel as such in the pipe wall. When an defect is encountered, part of the energy is reflected from the defect and, again, travels back along the original path to the transducer. At the transducer, a defect is noted when an electrical pulse or waveform is present at the transducer terminals. The processing of the results suffer the same difficulties as that discussed in the previous embodiment. There are considerable difficulties in discriminating the specific types of defects. Fundamentally, there are no reliable methods existing in the public domain.
The following are additional disadvantages of the second embodiment. First, the transducer angle must be maintained during the complete transmit/receive cycle; otherwise, the signal will not be detected even if a defect is present. Second, the pipeline must either be filled with a suitable liquid, all liquids are not suitable, for the couplant, or the device must be operated in a "slug" of liquid maintained between batching pigs, which is no small task. Third, neither of the techniques discussed above, filling the pipe with a suitable liquid, or operating in a "slug", are not readily acceptable to gas pipeline operators. Fourth, the large number of transducers lead to serious problems; since, transducer reliability leaves much to be desired and the maintenance can be onerous. Finally, the large number of transducers translates to a high quantity of data to be processed; hence, this system is infamous for creating huge data files, many gigabytes.
Consequently, based on the deficiencies of the prior art discussed above, a need has developed in the art for a defect detection system and method, which is inexpensive to implement, utilizes a minimum number of transducers, is capable of detecting defects in both liquid and gas piping, and can generate an enhanced characteristic representation of the defect. By generating an enhanced characteristic representation, the defect detection system and method facilitates discrimination of the defect.